I. Field
The following description relates generally to wireless communications, and more particularly to inter-cell interference management in a wireless communication system.
II. Background
A typical wireless communication network (e.g., employing frequency, time and code division techniques) includes one or more base stations that provide a coverage area and one or more mobile (e.g., wireless) terminals that can transmit and receive data within the coverage area. A typical base station can concurrently transmit multiple data streams for broadcast, multicast, and/or unicast services, wherein a data stream is a stream of data that can be of independent reception interest to a mobile terminal. A mobile terminal within coverage area of the base station can be interested in receiving one, more than one, or all data streams carried by the composite stream. Likewise, a mobile terminal can transmit data to the base station, other stations or other mobile terminals. Each terminal communicates with one or more base stations via transmissions on forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-single-out or a multiple-in-multiple-out (MIMO) system.
Conventional technologies utilized for transmitting information within a mobile communication network (e.g., a cell phone network) include frequency, time and code division based techniques. In general, with frequency division based techniques calls are split based on a frequency access method, wherein respective calls are placed on a separate frequency. With time division based techniques, respective calls are assigned a certain portion of time on a designated frequency. With code division based techniques respective calls are associated with unique codes and spread over available frequencies. Respective technologies can accommodate multiple accesses by one or more users.
With time division based techniques, a band is split time-wise into sequential time slices or time slots. Each user of a channel is provided with a time slice for transmitting and receiving information in a round-robin manner. For example, at any given time t, a user is provided access to the channel for a short burst. Then, access switches to another user who is provided with a short burst of time for transmitting and receiving information. The cycle of “taking turns” continues, and eventually each user is provided with multiple transmission and reception bursts.
Code division based techniques typically transmit data over a number of frequencies available at any time in a range. In general, data is digitized and spread over available bandwidth, wherein multiple users can be overlaid on the channel and respective users can be assigned a unique sequence code. Users can transmit in the same wide-band chunk of spectrum, wherein each user's signal is spread over the entire bandwidth by its respective unique spreading code. This technique can provide for sharing, wherein one or more users can concurrently transmit and receive. Such sharing can be achieved through spread spectrum digital modulation, wherein a user's stream of bits is encoded and spread across a very wide channel in a pseudo-random fashion. The receiver is designed to recognize the associated unique sequence code and undo the randomization in order to collect the bits for a particular user in a coherent manner.
More particularly, frequency division based techniques typically separate the spectrum into distinct channels by splitting it into uniform chunks of bandwidth, for example, division of the frequency band allocated for wireless cellular telephone communication can be split into 30 channels, each of which can carry a voice conversation or, with digital service, carry digital data. Each channel can be assigned to only one user at a time.
One commonly utilized variant is an orthogonal frequency division technique that effectively partitions the overall system bandwidth into multiple orthogonal subbands. Orthogonal meaning that the frequencies are chosen so that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. These subbands are also referred to as tones, carriers, subcarriers, bins, and frequency channels. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation) at a low symbol rate. Orthogonal frequency division has an advantageous ability to cope with severe channel conditions—for example, attenuation of high frequencies at a long copper wire, narrowband interference and frequency-selective fading due to multipath—without complex equalization filters. Low symbol rate makes the use of a guard interval between symbols affordable, making it possible to handle time-spreading and eliminate inter-symbol interference (ISI).
The orthogonality also allows high spectral efficiency, near the Nyquist rate. Almost the whole available frequency band can be utilized. OFDM generally has a nearly ‘white’ spectrum, giving it benign electromagnetic interference properties with respect to other co-channel users, and allowing higher transmit power when a single cell is considered alone. Also, without interior-carrier guard bands, the design of both the transmitter and the receiver is greatly simplified; unlike conventional FDM, a separate filter for each sub-channel is not required.
Orthogonality is often paired with frequency reuse, where communications taking place in cells located far apart may use the same portion of the spectrum, and ideally the large distance prevents interference. Cell communications taking place in nearby cells use different channels to minimize the chances of interference. Over a large pattern of cells, a frequency spectrum is reused as much as possible by distributing common channels over the entire pattern so that only far apart cells reuse the same spectrum. In such a case, and when scheduler flexibility to allocate bandwidth to different users is introduced, inter-cell interference control becomes critical.
With OFDM intra-cell interference is effectively contained. The roadblock to greater system efficiency becomes inter-cell interference. Methods to improve management of inter-cell interference, and at the same time be robust enough to handle various cell varieties (e.g., synchronous and asynchronous) and support a variety of user devices or end nodes (e.g., expensive, feature laden as well as economical, basic use), requires consideration in the wireless communications arena.